While typical owls (hereafter referred to simply as owls) vary greatly in size, with the smallest species, the elf owl, being a hundred times smaller than the largest, the Eurasian eagle-owl and Blakiston's fish owl, owls generally share an extremely similar body plan.[1] They tend to have large heads, short tails, cryptic plumage and round facial discs around the eyes. The family is generally arboreal (with a few exceptions like the burrowing owl) and obtain their food on the wing. The wings are large, broad, rounded and long. Like other birds of prey many owl species exhibit reverse sexual dimorphism in size, where females are larger than males (as opposed to the more typical situation in birds where males are larger).[2]

Because of their nocturnal habits they tend not to exhibit sexual dimorphism in their plumage. The feathers are soft and the base of each is downy, allowing for silent flight. The toes and tarsus are feathered in some species, and more so in species at higher latitudes.[3] Numerous species of owl in the genus Glaucidium and the northern hawk-owl have eye patches on the backs of their heads, apparently to convince other birds they are being watched at all times. Numerous nocturnal species have ear-tufts, feathers on the sides of the head that are thought to have a camouflage function, breaking up the outline of a roosting bird. The feathers of the facial disc are arranged in order to increase sound delivered to the ears. Hearing in owls is highly sensitive and the ears are asymmetrical allowing the owl to localise a sound. In addition to hearing owls have massive eyes relative to their body size. Contrary to popular belief, however, owls cannot see well in extreme dark and are able to see fine in the day.[1]

Owls are generally nocturnal and spend much of the day roosting. They are often perceived as tame since they will allow people to approach quite closely before taking flight, but they are instead attempting to avoid detection. The cryptic plumage and inconspicuous locations adopted are an effort to avoid predators and mobbing by small birds.

The supposed fossil heron"Ardea" lignitum (Late Pliocene of Germany) was apparently a strigid owl, possibly close to Bubo.[7] The Early–Middle Eocene genus Palaeoglaux from west-central Europe is sometimes placed here, but given its age it is probably better considered its own family for the time being.